There is considerable interest in producing electromagnetic radiation or particle beams that can propagate to distant targets or areas and act as probes or sensors of those targets. These methods are often referred to as active detection or active remote sensing methods in that the probing beam generates a signature that is stronger or of a different nature than the normal passive signatures. However, in many cases, the device that produces that probing radiation is large or fragile, and the radiation or particle beam cannot propagate easily to the target. This is particularly true of penetrating electromagnetic radiation beams (e.g., x-rays, terahertz (T-rays), gamma rays) or energetic particle beams (electron, ion, or neutron). To a lesser extent, this also applies to ultraviolet radiation since the radiation is heavily absorbed in the atmosphere.
Most of these forms of penetrating radiation require a high voltage (HV) source and produce the probe beam by converting a portion of the energy of a “drive” beam through some sort of conversion process. For example, x-ray beams are usually produced by having a high energy electron beam strike a metal foil, producing x-rays through the well-known bremsstrahlung process. In other cases, neutrons can be generated by having an ion beam strike a suitable thin target. The electron or ion drive beams in these cases may be produced by having an intense, ultrashort laser strike a solid target or a cluster.
Active detection, inspection, or sensing methods generally fall into two broad operational classes. The first of these is generally known as remote, or standoff, active detection, while the second is generally referred to as local active detection. The detection signatures usually fall into two broad classes: imaging and spectroscopy. X-rays, T-rays, and energetic particles penetrate into materials and can be used as illumination sources to generate surface and subsurface images of objects and areas of interest. Materials also have spectroscopic signatures in the terahertz frequency range that can be used to determine chemical composition. The presence of certain elements or isotopes can also be deduced through the products or nuclear reactions that are initiated by subjecting an object to a beam of hard x-rays, ions, or neutrons.
In remote or standoff active detection, a probe beam is directed to a target area some distance away, and a return signature is detected by one or more detectors. Because the return signature is often isotropic, in many cases the detectors are situated on a separate platform closer to the target. For penetrating probe beam radiation, a drive beam is usually required to produce the probe beam, with the drive beam and probe beam usually being produced on the same platform or on separate platforms in close proximity to one another where the location of the drive beam generator is fixed relative to the probe beam generator. In general, the intensity of the probe beam drops as it propagates to the target due to its intrinsic divergence and its interaction with the atmosphere, generally limiting its useful range to a few tens of meters or less. In addition, there is often substantial collateral ionizing radiation, so that in many cases, the practical limit on range or detection speed is determined not by the system size or performance but by the allowable radiation dose for collateral ionizing radiation. In some standoff active detection methods, the probing radiation is produced at or close to the target. One example of such a method is laser-induced breakdown spectroscopy (LIBS), in which radiation is produced by the breakdown plasma itself, where the LIBS plasma may be produced on the surface of a target or by generating a localized spark in the air near the target. However, this radiation does not penetrate the target.
There also are a variety of ultrashort pulse laser (USPL)-based schemes that generate probe and signature radiation from nonlinear effects. These schemes can be used to generate pulsed radiation in the infrared (IR), ultraviolet (UV), or THz regimes which can be useful for spectroscopic detection. However, although THz radiation can penetrate thin materials in some cases, these forms of radiation generally are not highly penetrating. In addition, none of these schemes produce highly penetrating probe beams (e.g., x-rays or energetic particles) that require large (>1017 W/cm2) laser intensities for their production.
For local active detection operations, the HV power source, drive beam generator, probe beam converter, and detectors are all in close proximity to the target. In some cases, these components are sufficiently compact and lightweight that they can be part of a small, mobile system that can be moved close to the target. In some such cases, the entire system can fit on a small, unmanned platform, while in others, the system is man-portable and can be placed by a trained operator or special forces member. However, the primary problem in these scenarios is that the strength and penetration capability of the probe beam generated on such small, mobile platforms is generally severely limited, and so these systems do not have broad utility.
There is, of course, another class of local active detection systems where the source and detectors are fixed in place, and the objects to be examined are brought to the scanner. In these systems, such as those used in airport screening or cargo inspection, the system can be relatively large, but control over the objects or vehicles to be examined is required. These systems have basically the same architecture as the small local systems described above.
In a third class of devices, the prime power and drive beam generator are located on a large, distant platform or at a fixed location, and the drive beam propagates to a small, remote probe beam converter located near the target. This class of systems may be referred to as “quasi-remote” since the drive beam generator is remote, but the probe beam converter is local with respect to the target area.
An obvious advantage of such quasi-remote systems is that the drive beam power and energy can be substantially larger than in the small mobile systems described above. Compared with the first class of standoff detection approaches, a quasi-remote system may produce a much more intense probe beam on the target or area to be examined and strongly reduce the amount of collateral ionizing radiation.
However, propagating the drive beam to the probe beam converter presents fundamental challenges. In a fixed laboratory, industrial, or hospital setting, the drive beam can be transported to the probe beam converter in a vacuum, using external magnetic fields to confine the drive beam. Such an architecture has been employed, for example, in large synchrotron light source facilities and in hospitals using proton therapy for cancer treatment. However, the particle beam optics and vacuum systems are extremely complex and expensive, making this approach far too cumbersome in most cases to be considered a practical approach in a more open environment or in cases where the target area or object to be inspected is not at a fixed location.
If the drive beam is an electron or ion beam, it can in principle be propagated through the air to the probe beam converter. However, scattering in the atmosphere causes massive spreading of the beam spot size or radius within a few tens of meters of propagation. Very high power electron or ion beams may be propagated through the air in a self-pinched state that slows beam expansion, but scattering and beam-plasma instabilities impose severe restrictions on range, and the collateral radiation dose is probably unacceptable. Although this approach theoretically is possible, there have not been any demonstrated cases in which an electron or ion beam has been propagated any significant distance to a probe beam converter in the open atmosphere.
Ultrashort pulse lasers (USPL) offer a potentially attractive source for the drive beam in a quasi-remote architecture since the pulses can be propagated over substantial distances in the atmosphere, and at high intensities, the pulses can in principle be converted to almost any form of penetrating radiation or particle beam. In many cases, the probe beam converter for a USPL-based system is much more compact than conventional systems producing a similar beam. For example, in laser wakefield accelerators (LWFA), the acceleration length is typically three orders or magnitude shorter than in conventional linear accelerators. However, the laser intensity during propagation through the air is limited by ionization and nonlinear effects to a level (<1012 W/cm2) that is orders of magnitude below that required to produce most forms of penetrating beams. The invention provides a method and approach to overcoming these limitations on the intensity of a propagating laser pulse using a quasi-remote architecture.
Quasi-remote detection systems may be thought of as a form of power beaming. In power beaming systems, energy from a large platform is transferred to a much smaller remote platform using a high energy laser or microwave beam. The typical goal is to provide sufficient energy to provide propulsion or power sensors or other energy consuming systems. The difference here is that for a laser drive beam, the intention is to produce very high peak power in a form that can be used to generate radiation or high energy particles while the total energy transported is generally modest.
There are a number of factors that can affect a laser pulse as it propagates through the air and degrade the ability to axially compress and focus the pulse. These include bulk processes such as absorption, scattering, and dispersion due to various molecular species, as well as interactions with aerosols. Other factors are processes that modify the refractive index of the air along the beam propagation path, such as laser induced processes (ionization, nonlinear focusing, thermal blooming) and naturally occurring fluctuations due to turbulence. These various effects are discussed below.
The laser beam can be scattered and absorbed by molecules in the air, especially water vapor molecules, and by aerosols. It turns out that for radiation in the 1 μm region of the spectrum aerosols are the predominant contributors to scattering and absorption. Taking the aerosol scattering coefficient βA≈0.3 km−1 the laser intensity drops by
      exp    ⁡          (                        -                      β            A                          ⁢        L            )        ≈      3    4  over a range L=1 km. See P. Sprangle, J. Peñano, and B. Hafizi, “Optimum Wavelength and Power for Efficient Laser Propagation in Various Atmospheric Environments,” Journal of Directed Energy 2, 71 (2006).
Air breakdown can cause ionization and plasma formation in the air, which in turn can modify the propagation characteristics of laser beams significantly. In the low intensity regime, breakdown proceeds via a multiphoton ionization process, while in the high intensity regime, tunneling ionization prevails. The corresponding ionization rates for n-photon ionization WMPI and tunneling ionization Wtun are given by equations (1) and (2) below:
                                                        W              MPI                        ⁡                          (              t              )                                =                                    α              n                        ⁢                                          I                n                            ⁡                              (                t                )                                                    ,                            (        1        )                                                                    W              tun                        ⁡                          (              t              )                                =                      4            ⁢                                                            Ω                  0                                ⁡                                  (                                                            U                      ion                                                              U                      U                                                        )                                                            5                ⁢                                  /                                ⁢                2                                      ⁢                                          E                H                                                                              E                  ⁡                                      (                    t                    )                                                                                        ⁢                          exp              ⁡                              [                                                      -                                          2                      3                                                        ⁢                                                            (                                                                        U                          ion                                                                          U                          U                                                                    )                                                              3                      ⁢                                              /                                            ⁢                      2                                                        ⁢                                                            E                      H                                                                                                            E                        ⁡                                                  (                          t                          )                                                                                                                                          ]                                                    ,                            (        2        )            where I(t) is the laser intensity, αn and Ω0 are given coefficients, Uion(UH) is the ionization potential of the molecule under consideration (hydrogen), EH is the hydrogenic electric field and E(t) is the laser electric field. See P. Sprangle, J. Peñano and B. Hafizi, “Propagation of intense short laser pulses in the atmosphere,” Phys. Rev. E 66, 046418 (2002) (“Sprangle 2002”); P. Sprangle, J. R. Peñano, A. Ting, B. Hafizi and D. F. Gordon, “Propagation of Short, High-Intensity Laser Pulses in Air,” Journal of Directed Energy 1, 73 (2003) (“Sprangle 2003”) P. Sprangle, A. Ting, J. Peñano, R. Fischer and B. Hafizi, IEEE J. Quantum Electron. 45, 138 (2009) (“Sprangle 2009”); and N. B, Delone and V. P, Krainov, “Tunneling and barrier-suppression ionization of atoms and ions in a laser radiation field,” Physics—Uspekhi 41 (5) 469-485 (1998).
By far the most abundant constituents of the atmosphere are N2 and O2. Although nitrogen molecules are nearly four times more abundant than oxygen molecules, the dominant air breakdown process is photoionization of O2 since it is more readily ionizable (i.e., UO2=12.1 eV, UN2=15.6 eV). Based on these expressions it turns out that a ˜ns laser pulse can propagate with little ionization so long as its intensity is ≦1013 W/cm2 
Another process affecting the propagation of a laser pulse as travels through the atmosphere is spreading of the pulse in time due to dispersion. A laser pulse of finite (initial) duration T0 spreads as it propagates in the atmosphere as a result of the spread in the group velocity associated with the wavenumbers in the pulse. The distance over which an initially unchirped pulse nearly doubles in duration is given by ZT0=T02/2|β2|, where β2 is the group velocity dispersion parameter. Noting that β2≈2.2×10−31s2/cm for air at STP, it follows that for a 100 fsec pulse, ZT0 is on the order of a few hundred meters, which is comparable to the ranges of interest here. Thermal blooming can be neglected since the time scales for heating of air and the resulting hydrodynamic flows are much longer than the duration of laser pulses of interest.
However, a frequency chirp imposed on the pulse also will affect its spreading. In the absence of nonlinear effects and in an homogeneous atmosphere, the pulse duration T(z) will vary with propagation distance z according to
                              T          ⁡                      (            z            )                          =                                                            T                0                            ⁡                              [                                                                            (                                              1                        +                                                                              β                            ⁡                                                          (                              z                              )                                                                                ⁢                                                      z                                                          Z                                                              T                                0                                                                                                                                                        )                                        2                                    +                                                            (                                              z                                                  Z                                                      T                            0                                                                                              )                                        2                                                  ]                                                    1              ⁢                              /                            ⁢              2                                .                                    (        3        )            See Sprangle 2002, Sprangle 2003, and Sprangle 2009, supra. This expression applies to a pulse whose amplitude has a Gaussian variation proportional to exp[−(1+iβ)(t−z/vg)2/T2] in the pulse variable t−z/vg, where Vg is the pulse group velocity and β(z) is the chirp parameter defined such that the instantaneous frequency spread along the pulse is given by 2β(z)(t−z/vg)2/T2(z). The full frequency chirp over the pulse duration [−T, T] is given by δωfull=4β/T. For a negative chirp parameter that is sufficiently large (in magnitude), the pulse duration can be made to decrease, i.e., pulse compression can take place.
In addition, air is a nonlinear medium with a refractive index that is weakly dependent on the laser intensity I. That is, n=n0+n2I, where n0−1≈2×10−4 at STP and n2≈3×10−19 cm2/W for ˜ns laser pulses. This effect, known as the optical Kerr effect, is due to the nonlinear polarization of the bound electrons. One manifestation of the optical Kerr effect is self-focusing of a laser beam if the power exceeds a certain threshold, Pk≡λ2/(2πn0n2), which is ≈3GW for air at STP and a laser wavelength λ=1 μm. Another nonlinear polarization effect arises from the dumb-bell shape of the diatomic molecules N2 and O2. The induced polarization of these molecules in the laser electric field leads to a rotational Raman effect that is of the same order of magnitude as the Kerr effect. Both of these effects can be avoided provided the power in the laser beam is well below 3 GW.
A powerful laser beam propagating in air can undergo filamentation. Growth of the instability associated with filamentation is expressed in terms of the B-integral, which can be expressed as B≈k L (n2/n0)I, where k is the wavenumber, L is the propagation range, n0≈1 and n2≈3×10−19cm2/W are the linear and nonlinear refractive indices, respectively, and I is the intensity. For a propagation range of 300 m it is necessary for the intensity to be <1010 W/cm2 to avoid filamentation of the laser pulse.
Atmospheric turbulence can be the most important process limiting the ability to compress and focus the pulse to high intensities. Turbulence degrades the phase fronts and leads to local hot spots. The instantaneous beam spot size increases (spreads) while the beam centroid wanders.
The propagation of laser beams in the atmosphere can be divided into the weak or strong turbulence regimes. Laser beam propagation characteristics, such as the spot size and centroid wander, depend on the turbulence regime, characterized by the Rytov varianceσR2=1.23Cn2L11/16λ−7/6  (4)where λ is the wavelength, L is the propagation range, and Cn2 is the refractive index structure constant which is a measure of the turbulence level. See L. C. Andrews, R. L. Phillips, Laser Beam Propagation through Random Media, 2nd Ed., SPIE Press, Bellingham, Wash., 2005, p. 263; J. W. Hardy, Adaptive Optics for Astronomical Telescopes, Oxford University Press, New York, N.Y., 1998, p. 89; and R. L. Fante, “Electromagnetic beam propagation in turbulent media,” IEEE Proceedings 63, 1669 (1975).
Weak turbulence is characterized by σR2<1. For the propagation ranges of interest here the Rytov variance is typically less than the unity. For a collimated beam in this limit, the characteristic displacement of the beam centroid, which is termed the beam wander, is given by
                              ρ          w          2                =                              2.97            ⁢                          L              2                                                          k              2                        ⁢                          ρ              0                              5                ⁢                                  /                                ⁢                3                                      ⁢                          D                              1                ⁢                                  /                                ⁢                3                                                                        (        5        )            where k is the wavenumber, D=√{square root over (2)}R0 is the aperture diameter, R0 is the Gaussian spot size of the laser beam and ρ0=0.158(λ2/Cn2L)3/5. Wandering of the beam centroid takes place on a time scale defined by the Greenwood frequency fG, where fG≈Vwind/ρ0 and Vwind is the wind speed. When the exposure (e.g., of a camera) is longer than the atmospheric coherence time 1/fG, the spot size is effectively larger than the actual, instantaneous (i.e., short-term), spot size. For a collimated beam the short-term beam spreading is given by
                              ρ          s          2                ≈                                            M              2                        ⁢                                          4                ⁢                                  L                  2                                                                              k                  2                                ⁢                                  D                  2                                                              +                                    D              2                        4                    +                                                                      4                  ⁢                                      L                    2                                                                                        k                    2                                    ⁢                                      ρ                    0                    2                                                              ⁡                              [                                  1                  -                                      0.62                    ⁢                                                                  (                                                                              ρ                            0                                                    D                                                )                                                                    1                        ⁢                                                  /                                                ⁢                        3                                                                                            ]                                                    6              ⁢                              /                            ⁢              5                                                          (        6        )            where M2 is the intrinsic beam quality. Note that ρs=D/2=R0/√{square root over (2)} at L=0. From Equation (6), the effective turbulence-corrected beam quality is given by
                              M          eff          4                =                              M            4                    +                                                                      D                  2                                                  ρ                  0                  2                                            ⁡                              [                                  1                  -                                      0.62                    ⁢                                                                  (                                                                              ρ                            0                                                    D                                                )                                                                    1                        ⁢                                                  /                                                ⁢                        3                                                                                            ]                                                    6              ⁢                              /                            ⁢              5                                                          (        7        )            
In summary, atmospheric turbulence causes fluctuations in the refractive index of the air and leads to spreading of the laser beam and wandering of the beam centroid. Both of these increase with increasing range and with increasing level of turbulence. These effects can be compensated for by employing adaptive optics techniques which, however, can be cumbersome and expensive. The alternative is to choose a propagation range that is short enough to give acceptable results.